1. Field of the Invention
The present invention relates to an ink jet recording apparatus which ejects ink droplets towards to a recording medium in response to electric drive pulses.
2. Description of the Related Art
Ink jet recording apparatuses have become popular in recent years due to their numerous merits, including quiet operation while printing, the ability to print at high speed and the ability to use low-cost plain paper. The ink-on-demand type ink jet recording apparatus (in which ink is ejected only when printing is required) has become the most common type of ink jet printer because it is not necessary to retrieve ink not used for printing.
A conventional ink-on-demand ink jet recording apparatus is described in Japanese Laid-Open Application JP-A-79171/1980. This recording apparatus provides plural electrostrictive distortion bodies (piezoelectric devices), which are linked to each ejection chamber. When activated by digital electric pulse, the piezoelectric element mechanically distorts one or more walls of its respective ejection chamber, to momentarily increase pressure inside the ejection chamber and force expulsion of an ink drop. One can actually control the volume of ink drop emitted as well, by controlling the length or magnitude of the driving pulse. Thus, a good gradient image can be obtained using the above-described ink jet head by simple application of a requisite pulse information to the desired piezoelectric elements contained in the head, at least in theory.
With this type of conventional ink jet recording apparatus, however, it is extremely difficult and time-consuming to affix the piezoelectric device to the ejection chamber, thus making manufacturing difficult and prone to error. Plus, in practice, the thickness of conventionally manufactured piezoelectric devices tends to vary greatly, and the thickness of the adhesive applied to attach them to a chamber wall also fluctuates. Together, these factors produce undesirable scattering in ejection chamber responsiveness, which can degrade output and even shorten the life of the head. Thus, it is, in fact difficult to precisely control the size of the ink droplets according to the gradient signal.
In addition, the drive voltage required to obtain an enough deflection increases as the size of the piezoelectric device (especially unimorphic piezoelectric devices) decreases, and it is therefore difficult to form small electrostrictive bodies, mount them in a high density package, and drive the electrostrictive bodies with a relatively low drive voltage. Therefore, using piezoelectric device technology in high density, multiple nozzle ink jet head implementations suitable for high resolution gradient image printing without high drive voltages is exceedingly difficult.
It is, therefore an object of the present invention to provide an ink jet recording apparatus for printing high resolution gradient images using a low drive voltage by easily and precisely controlling the ink ejection volume according to a digital gradient signal describing the gradient of each pixel.